The present invention is directed generally to a DC voltage converting device for converting a DC voltage, and more particularly, to a DC voltage converting device for effecting a conversion from a relatively low DC power supply into a higher DC voltage.
The DC voltage converting device, i.e., a DC--DC converter, is employed for obtaining plural kinds of DC voltages. One typical example may be a DC--DC converter reported on p. 311 of Practical Electronic Circuit Handbook (published by CQ Publishing Co., Ltd), which is depicted in FIG. 5.
This type of DC--DC converter defined as a fundamental flyback type power supply circuit is composed of a charge coil L, transistors Tr2 and Tr3, resistors r1 to r3, capacitors c1 and c2, a diode DD and a Zener diode ZD.
Based on this construction, the charge coil L, the transistor Tr1, the resistors r1 and r2 and the capacitor c1 are combined to contitute an oscillation circuit. The transistor Tr2 is used as a switching element for controlling a charge and a discharge of the charge coil L. More specifically, when turning ON the transistor Tr2, the charge coil L is charged with a charge current i1 flowing therethrough. Whereas in the case of turning OFF the transistor Tr2, a discharge current i2 flows via the diode DD and is charged in the capacitor c2. An output voltage is stepped up by repeating such operations. If the voltage exceeds a voltage of the Zener diode ZD, however, feedback is applied to the transistor Tr3, with the result that a bias voltage of the transistor Tr2 is bypassed to stop the oscillations. Consequently, the capacitor c2 is charged with no electricity, and the output is reduced. When the output voltage is decreased under the voltage of the Zener diode ZD, the oscillations resume. It is therefore possible to obtain an output voltage substantially equal to the voltage of the Zener diode ZD.
In the above-described DC--DC converter, when the output voltage exceeds the voltage of the Zener diode ZD, the oscillations cease. Whereas if lower than the Zener diode voltage, the oscillations resume. The output voltage is thus kept to a constant value. At that time, however, the Zener diode ZD and the transistors Tr1 and Tr2 come into a linear region, which in turn causes parasitic oscillations due to noise components and a heat emission or a heat breakdown of the transistor Tr2. To prevent these phenomena, a capacitor CC for a filter may be interposed in a voltage feedback loop. This arrangement has, however, proven ineffective because of a deterioration of respondency to fluctuations in the output voltage.